This invention relates generally to methods for generating fluid droplets from a reservoir of immiscible fluids using focused acoustic energy. More particularly, the invention relates to methods for forming one or more features on a substrate surface from one or more reservoirs of immiscible fluid.
It is often desirable to generate droplets of a fluid mixture composed of substances that are normally immiscible with one another, wherein one or more of the components may be viscous. Using prior methods for generating droplets of viscous fluids, however, it has been difficult to achieve control over droplet size. For example, conventional inkjet technologies, such as piezoelectric and thermal inkjet printing, have been limited by the need to force a viscous material through a small nozzle. In order to dispense quantities of fluid on the order of 1 picoliter in volume, nozzle openings with dimensions of under 30 micrometers would be required. The energy required to move a viscous fluid out of such a small nozzle opening would be great and would likely result in substantial shearing of the fluidic material. Nozzle clogging also poses a problem with these types of printing technologies.
For a thermal inkjet printer, the ejection energy comes from the vaporization of the fluid to be printed. Most viscous fluids have an extremely high boiling point and would require significant thermal energy input. Piezoelectric printing might be a more efficient way of ejecting droplets of viscous material, but the thermal energy required would still be substantial. For example, U.S. Pat. No. 5,229,016 describes a method for dispensing solder with a piezoelectric ejection device. The system requires elevated temperature and a backpressure system of 30 psi in order to eject solder through a 25 micrometer orifice. Even with pressure assist, the maximum ejection rate for solder is on the order of 10 kHz. In U.S. Pat. No. 5,498,444, a method is described for ejecting polymers using a piezoelectric ejection device that operates at up to 40 cps, along with elevated temperatures. The need for elevated temperatures, of course, reduces the number of materials with which one can work, as heating many materials in order to reduce viscosity can result in degradation. Other devices for producing droplets of fluids, such as described, for example, in U.S. Pat. No. 4,812,856 to Wallace et al., are disadvantageous in various respects as well, including slow repetition rate due to refill time.
Use of focused acoustic energy in printing technology is known in the art. For example, U.S. Pat. No. 4,308,547 to Lovelady et al. describes a liquid drop emitter that utilizes acoustic principles in the ejection of liquid from a body of liquid onto a moving document to form characters or bar codes thereon. Specifically, Lovelady et al. is directed to a nozzleless inkjet printing apparatus, wherein controlled drops of ink are propelled by an acoustical force that is produced by a curved transducer at or below the surface of the ink. In contrast to inkjet printing devices, nozzleless fluid ejection devices (as described in the aforementioned patent) are not prone to clogging and the associated disadvantages, e.g., misdirected fluid or improperly sized droplets.
The development of nozzleless fluid ejection, however, has generally been limited to ink printing applications. Since the development of ink printing technology is strongly influenced by economic concerns, the bulk of development efforts have been concentrated on reducing printing costs and improving printing speed, rather than on enhancing print quality. For example, U.S. Pat. No. 5,087,931 to Rawson is directed to a system for transporting ink under constant flow to an acoustic ink printer having a plurality of ejectors aligned in an axis, each ejector associated with a free surface of liquid ink. Having a plurality of ejectors generally increases printing speed. However, it is more difficult to control fluid ejection, specifically droplet placement, when a plurality of ejectors is used in place of a single ejector.
As another example, U.S. Pat. No. 4,797,693 to Quate describes an acoustic ink printer for printing polychromatic images on a recording medium. The printer is described as comprising a combination of a carrier that contains a plurality of differently colored liquid inks, a single acoustic printhead acoustically coupled to the carrier for launching converging acoustic waves into the carrier, an ink transport means to position the carrier to sequentially bring the differently colored inks into alignment with the printhead, and a controller to modulate the radiation pressure exerted against the inks. It has been disclosed that this type of printer is designed with cost effectiveness in mind. However, this device can eject only a limited quantity of ink from the carrier before the liquid surface moves out of acoustic focus and drop ejection ceases.
The technique of using unfocused acoustic energy to generate droplets of immiscible fluids is also known in the art. For example, U.S. Pat. No. 4,801,411 to Wellinghoff et al. is generally directed to the production of ceramic particles. In particular, the patent describes that ultrasonic energy may be employed to produce an aerosol spray of two-layer droplets. The technique involves using unfocused ultrasonic energy to effect cavitation in a reservoir containing a stable bilayer of two immiscible liquids in order to form the two-layer droplets. As a result, the technique produces a spray of particles of random size, thus failing to provide the level of control over droplet size and direction that characterizes technologies using focused acoustic energy to produce uniform droplets.
Thus, there is a need in the art for improved systems that make use of focused acoustic ejection technology in combination with reservoirs of immiscible fluids to generate fluid droplets, but without the disadvantages associated with inkjet printing devices relying on a nozzle for droplet ejection, and other prior acoustic ejection systems. In addition, it has recently been discovered that use of immiscible fluids provides additional benefits previously unknown in the art of acoustic ejection, particularly in the field of array preparation.
In one aspect, then, the invention relates to a method for forming a feature on a substrate surface. The method involves providing a fluid-containing reservoir containing at least two immiscible fluids and a substrate surface, porous or nonporous, in droplet-receiving relationship to the fluid-containing reservoir. Focused acoustic energy is applied in a manner effective to eject a droplet of fluid from the reservoir such that the droplet is deposited on the substrate surface, thereby forming a feature thereon. The ejected droplet is comprised of at the least two immiscible fluids. As a result, the feature formed on the substrate surface may comprise one fluid circumscribed by another. While features of any size may be formed, the circumscribed fluid typically has a diameter of about 2 to about 200 micrometers. Preferably, the circumscribed fluid has a diameter of about 5 to about 50 micrometers.
Any of a number of types of fluids may be used. Typically, one of the fluids is aqueous and another of the fluids is nonaqueous. When the invention is used in biomolecular applications, either the aqueous or nonaqueous fluids may contain a biomolecule. In some embodiments, the biomolecule may be selected from the group consisting of DNA, RNA, antisense oligonucleotides, peptides, proteins, ribosomes, and enzyme cofactors. In addition or in the alternative, the biomolecule may be a pharmaceutical agent. In either case, one or more dyes may be used. For example, each of the aqueous and nonaqueous fluids may contain a different fluorescent and/or chemiluminescent dye. Furthermore, either the aqueous or the nonaqueous fluid may contain a surface-modifying material capable of altering the wetting properties of the substrate surface. In such a case, fluid may be evaporated from a deposited feature to allow the surface-modifying material to alter the wetting properties of the substrate surface at the location of the feature.
The nonaqueous fluid may contain a number of organic materials. In some instances, the organic material is selected from the group consisting of hydrocarbons, halocarbons, hydrohalocarbons, haloethers, hydrohaloethers, silicones, halosilicones, and hydrohalosilicones. In addition or in the alternative, the organic material may be lipidic and optionally selected from the group consisting of fatty acids, fatty acid esters, fatty alcohols, glycolipids, oils, and waxes. In some instances, at least two fluids may be organic.
Typically, the reservoirs contain a lower layer comprised of a first fluid, and an upper layer comprised of a second fluid, wherein the first and second fluids are immiscible. As a whole, the lower layer may exhibit a greater thickness than the upper layer. In addition or in the alterative, the second fluid may have a higher vaporization temperature than the first fluid. When the upper layer has a nonuniform thickness, focused acoustic energy may be applied to the reservoir to eject a droplet comprised of a predetermined volume of the first fluid through the upper layer at an aperture region that exhibits a local thickness minimum.
In such a case, the lower layer under the aperture region of the upper layer typically exhibits a greater thickness than the local thickness minimum of the upper layer. The local thickness minimum may be no more than about 10% of the thickness of the lower layer under the aperture region. In some instances, the aperture region of the upper layer may be a molecular bilayer or molecular monolayer. In the most extreme case, the aperture region is a hole that extends through the upper layer. In order to ensure that the aperture region has a sufficiently large cross-sectional area for droplets to be ejected therethrough, the aperture region typically has a diameter of at least about twice, but not more than 10 times, that of the ejected droplet.
In some instances, the spatial relationship between the substrate surface and the reservoir is altered. By applying focused acoustic energy in a manner effective to eject another droplet of fluid from the reservoir such that the droplet is deposited on the substrate surface at another site, a plurality of features may be formed on the substrate surface. When the application of focused acoustic energy is coordinated with the movement of the substrate surface with respect to the reservoir, an array of substantially identical features may be formed on the substrate surface.
The invention also relates to a method for forming an array of features on a substrate surface. When different feature types are desired in the array, a plurality of fluid-containing reservoirs may be provided, each containing at least two immiscible fluids. Although the fluid content of each reservoir is typically different from the other(s), each reservoir may contain at least one fluid in common. By placing the substrate surface in droplet-receiving relationship to the fluid-containing reservoirs and applying focused acoustic energy in a manner effective to eject a droplet of fluid from each of the reservoirs, the droplets are deposited at different sites on the substrate surface. As a result, an array of different features may be formed on the substrate surface. In such a case, each ejected droplet is comprised of at the least two immiscible fluids contained in the fluid-containing reservoir from which the droplet is ejected.
In some instances, the droplets are deposited successively. This is typically the case when a single ejector is used. In other instances, the droplets are deposited simultaneously.
The invention further relates to a method for reducing the size of a feature formed by a device that creates a feature of a specific nominal size on a substrate. The method involves the use of a device that includes a reservoir adapted to contain a feature-forming fluid and an ejector for ejecting a fluid droplet of a predetermined volume from the reservoir. The ejector includes an acoustic radiation generator for generating acoustic radiation, and a focusing means for focusing the generated acoustic radiation. When the ejector is positioned in acoustic coupled relationship to the reservoir, the focusing means directs the acoustic radiation at a focal point sufficiently near the fluid surface in the reservoir so as to eject and deposit the droplet on a substrate. As a result, the device is capable of forming a feature of the desired nominal size on the substrate surface, wherein the nominal size corresponds to the predetermined volume.
Instead of filling the reservoir with a single fluid, the reservoir is filled with a feature-forming fluid and an additional fluid that is immiscible with the feature-forming fluid. Once substrate is provided in droplet-receiving relationship to the reservoir, the device is activated to generate focused acoustic energy in a manner effective to eject a droplet of the predetermined volume and comprised of the immiscible fluids from the reservoir. As a result, the ejected droplet is deposited on the substrate surface, thereby forming a feature of the feature-forming fluid on the substrate. Because the feature-forming fluid occupies only a portion of the predetermined droplet volume, the formed feature is smaller than the nominal feature size.